Figure 9.

3-D visualization of an ∼250-nm wide subunit spanning a fraction of the chromatid width supports a hierarchical folding model. (A) Dependence of labeled band width versus insert size reveals folding subunits. In a simple radial loop model (top), as the insert size increases, the labeled region spans an increasing fraction of the chromatid cross section, with the minimal width of a labeled band corresponding roughly to the diameter of a 30-nm chromatin fiber loop. With a successive coiling model, as the insert region increases in size, the labeled region spans an increasing fraction of the chromatid cross section, but the width of this labeled region does not increase until it spans a full chromatid cross section (bottom). The width of the minimal labeled region corresponds to the diameter of the folding subunit, significantly larger than a 30-nm chromatin loop. (B) Reconstructed orthogonal cross sections of EM serial section data built with NewVision. Arrows and arrowhead show the same nanogold-labeled areas as in Fig. 8, I–L. The spot-like labeled region appears as a labeled band extending across the chromatid in the orthogonal view. Thick red, green, and blue lines in images a, b, and c are, respectively, X, Y, and Z axes of a left orthogonal system with origin inside the nanogold-labeled insert. (C) Solid model display for same chromosome region shown in Fig. 8, I–L and Fig. 9 B reveals similar appearance of labeled regions on both chromatids. In the original serial sections, one region appeared as a band (Fig. 8 I, arrowhead) and one as a spot (Fig. 8, J and K, arrows). In the orthogonal views in B and in the solid model, both regions appear as ∼250–300-nm wide segments spanning only a fraction of the chromatid cross section.